Rotary drag bits have been use for subterranean drilling for many decades, and various sizes, shapes and patterns of natural and synthetic diamonds have been used on drag bit crowns as cutting elements. A drag bit can provide an improved rate of penetration (ROP) over a tri-cone bit in many formations.
Over the past few decades, rotary drag bit performance has been improved with the use of a polycrystalline diamond compact (PDC) cutting element or cutter, comprising a planar diamond cutting element or table formed onto a tungsten carbide substrate under high temperature and high pressure conditions. The PDC cutters are formed into a myriad of shapes including circular, semicircular or tombstone, which are the most commonly used configurations. Typically, the PDC diamond tables are formed so the edges of the table are coplanar with the supporting tungsten carbide substrate or the table may overhang or be undercut slightly, forming a “lip” at the trailing edge of the table in order to improve the cutting effectiveness and wear life of the cutter as it comes into formations being drilled. Bits carrying PDC cutters, which for example, may be brazed into pockets in the bit face, pockets in blades extending from the face, or mounted to studs inserted into the bit body, have proven very effective in achieving an ROP in drilling subterranean formations exhibiting low to medium compressive strengths. The PDC cutters have provided drill bit designers with a wide variety of improved cutter deployments and orientations, crown configurations, nozzle placements and other design alternatives previously not possible with the use of small natural diamond or synthetic diamond cutters. While the PDC cutting element improves drill bit efficiency in drilling many subterranean formations, the PDC cutting element is nonetheless prone to wear and damage when exposed to certain drilling conditions, resulting in a shortened life of a rotary drag bit using such cutting elements.
Thermally stable diamond (TSP) is another type of synthetic diamond, PDC material which can be used as a cutting element or cutter for a rotary drag bit. TSP cutters, which have had catalyst used to promote formation of diamond-to-diamond bonds in the structure removed therefrom, have improved thermal performance over PDC cutters. The high frictional heating associated with hard and abrasive rock drilling applications creates cutting edge temperatures that exceed the thermal stability of PDC whereas TSP cutters remain stable at higher operating temperatures. This characteristic also enables them to be furnaced into the face of a matrix-type rotary drag bit.
While the PDC or TSP cutting elements provide better ROP and manifest less wear during drilling as compared to some other cutting element types, it is still desirable to further the life of rotary drag bits and improve cutter life regardless of the cutter type used. Either type of PDC cutting element is generally fixedly mounted to a rotary drill bit that cuts the formation substantially in a shearing action through rotation of the bit and application of drill string weight thereto. A plurality of either, or even both, types of PDC cutting elements is mounted on a given bit, and cutting elements of various sizes may be employed on the same bit.
Drill bit bodies may be cast and/or machined from metal, typically steel, or may be formed of a powder metal infiltrated with a liquid binder at high temperatures to form a matrix-type bit body. PDC cutting elements may be brazed to a matrix-type bit body after furnacing, or TSPs may even be bonded into the bit body during the furnacing process used for infiltration. Cutting elements are typically secured to cast or machined (steel body) bits by preliminary bonding to a carrier element, commonly referred to as a stud, which in turn is inserted into an aperture in the face of the bit body and mechanically or metallurgically secured thereto. Studs are also employed with matrix-type bits, as are cutting elements secured via their substrates to cylindrical carrier elements affixed to the matrix-type bit body.
It has long been recognized that PDC cutting elements, regardless of their method of attachment to drag bits, experience relatively rapid degradation in use due to the extreme temperatures and high loads, particularly impact loading, during drilling. One of the major observable manifestations of such degradation is the fracture or spalling of the PDC cutting element cutting edge, wherein large portions of the superabrasive PDC layer separate from the cutting element. The spalling may spread down the cutting face of the PDC cutting element, and even result in delamination of the superabrasive layer from the backing layer of substrate, or from the bit itself if no substrate is employed. At the least, cutting efficiency is reduced by cutting edge damage, which also reduces the rate of penetration of the drag bit into the formation. Even minimal fracture damage can have a negative effect on cutter life and performance. Once the sharp corner on the leading edge (taken in the direction of cutter movement) of the diamond table is chipped, the amount of damage to the table continually increases, as does the normal force required to achieve a given depth of cut. Therefore, as damage to the cutting edge and cutting face occurs and the rate of penetration of the drag bit decreases, the conventional rig-floor response of increasing weight on bit quickly leads to further degradation and ultimately catastrophic failure of the chipped cutting element.
While continuing to develop and seek out improvements for longer lasting cutters and improvements to cutter performance, it would be desirable to utilize or take advantage of the nature of cutting element damage in extending or improving the life of the drag bit by reducing cutting element damage due to impact loading.
One approach to enhancing bit life is to use the so-called “backup” cutter to extend the life of a primary cutter of the drag bit particularly when subjected to dysfunctional energy or harder, more abrasive, material in the subterranean formation. Conventionally, the backup cutter is positioned in a second cutter row, rotationally following in the path of a primary cutter, so as to engage the formation should the primary cutter fail or wear beyond an appreciable amount. The use of backup cutters has proven to be a convenient technique for extending the life of a bit, while enhancing stability without the necessity of designing the bit with additional blades to carry more cutters which might potentially comprise bit hydraulics due to reduced available fluid flow area over the bit face and less-than-optimum fluid flow due to unfavorable placement of nozzles in the bit face. Durability may be quantified in terms of cutter placement, and in terms of the ability to maintain the sharpness of each cutter for a longer period of time while drilling. In this sense, “sharpness” of each cutter involves improving wear of the diamond table, including less fracturing, chipping or damage to the diamond table caused by point loading, dysfunctional energy, or drill string bounce.
Accordingly, there is an ongoing desire to improve or extend rotary drag bit life and performance in any type of subterranean formation type being drilled. There is a further desire to extend the life of a rotary drag bit by beneficially orienting and positioning cutters upon the bit body to have greater support of the rotary drag bit to protect the cutting element from excessive impact and torsional loading to prevent fracturing and chipping of the cutting element.